The present invention relates to the use of photocatalysis, and more particularly to the use of a photocatalytic reactor to remove metals from industrial waste streams.
Using semiconductors, such as titanium dioxide (TiO.sub.2), as a catalyst when activated by light, especially ultraviolet light, is an effective advanced oxidation process that is sometimes known as photocatalysis. A semi-conductive material has a valence band filled with electrons and an empty conductive band. When semiconductors with an appropriate energy difference between the two bands are irradiated by light, electrons in the valence bands are excited to the conductive bands leaving holes behind (for TiO.sub.2 ultraviolet light will excite electrons to transfer to the conductive bands). The holes thus created can extract electrons from surrounding chemical species to form free radicals, which are strong oxidants giving photocatalytic processes the potential for fast and complete organic destruction or metal recovery. Photocatalysis is capable of destroying a wide variety of hazardous organic contaminants, such as polyaromatic hydrocarbons, and many pesticides, and is capable of recovering many metals, such as silver, gold, copper, mercury, platinum, from waste streams.
There is increasing interest in using titanium dioxide activated by ultraviolet light to recover metals in a number of applications, such as the recovery of silver from photography processing wastes. However, there are several drawbacks in the use of titanium dioxide in colloidal form in photocatalysis for metal recovery. It is difficult to separate the recovered metal from the titanium dioxide slurry; there is low efficiency or quantum yield of the photo nucleation, deposition and growth of the metal at its reaction site; free radicals scavengers and electron-hole recombination reduce efficiency; and high concentrations of colloidal suspensions create shadow effects which reduce light intensity.
Some of the difficulties of using colloidal titanium dioxide have been relieved by the use of porous TiO.sub.2 -SiO.sub.2 glass-ceramic semiconductor material or low-reflectivity, porous, metal oxide coatings. For example, separation is easier than from a slurry. Several methods for making such a porous glass-ceramic have been developed. One method that uses heat treatment and subsequent acid leaching is taught by Kokura and Yamane in Preparation of porous Glass-Ceramics of the TiO.sub.2 System, Journal of Material Science 20, 4309 (1985). This method results in TiO.sub.2 being imbedded on a porous SiO.sub.2 skeleton. Methods of depositing transparent metal oxide coatings are discussed by S. P. Mukherjee in Ultrastructure of Ceramics, Glasses, and Composites, Hench, L. L. and D. R. Ulrich Eds., John Wiley and Sons, New York, 1984. The use of porous glass-ceramic membranes in a reaction vessel for photocatalytic reactions is taught in U.S. Pat. No. 5,137,607.